Eärnur had held the crown only seven years when the Lord of Morgul repeated his challenge, taunting the king that to the faint heart of his youth he had now added the weakness of age. Then Mardil could no longer restrain him, and he rode with a small escort of knights to the gate of Minas Morgul. None of that riding were ever heard of again. It was believed in Gondor that the faithless enemy had trapped the king, and that he had died in torment in Minas Morgul; but since there were no witnesses of his death, Mardil the Good Steward ruled Gondor in his name for many years.

Now the descendants of the kings had become few. Their numbers had been greatly diminished in the Kin-strife.... So it was that no claimant to the crown could be found who was of pure blood, or whose claim all would allow; and all feared the memory of the Kin-strife, knowing that if any such dissension arose again, then Gondor would perish. Therefore, though the years lengthened, the Steward continued to rule Gondor, and the crown of Elendil lay in the lap of King Eärnil in the Houses of the Dead, where Eärnur had left it.

The Lord of the Rings, Appendix A, Annals of the Kings and Rulers: The Realms in Exile

Earnur... went to war with Minas Ithil and Mordor and never returned; nor was his body ever recovered. Some said he was carried off alive by the evil king. He left no children. No male descendants of clear title (or nearly pure blood) of Elendil could be discovered. Mardil the Steward, grandson of Pelendur, governed nominally 'until the King's return', and this became an habitual formula.

The Peoples of Middle-Earth, HoME Vol 12, Part 1, Ch 7, The Heirs of Elendil: The Ruling Stewards of Gondor